1. Field of the Invention
The present invention relates to a mixture of a plurality of different titanyltetraazaporphyrin compounds, and an electrophotographic photoconductor comprising a photoconductive layer in which the above-mentioned mixture of the titanyltetraazaporphyrin compounds is contained as the photoconductive material.
2. Discussion of Background
Conventionally, the photoconductive material for use in the electrophotographic process is roughly divided into two groups, that is, an inorganic photoconductive material and an organic photoconductive material. The above-mentioned electrophotographic process is one of the image forming processes, through which the surface of the photoconductor is charged uniformly in the dark to a predetermined polarity, for instance, by corona charge. The uniformly charged photoconductor is exposed to a light image to selectively dissipate the electric charge of the exposed area, so that a latent electrostatic image is formed on the photoconductor. The thus formed latent electrostatic image is developed into a visible image by use of a toner comprising a coloring agent such as a dye or pigment, and a polymeric material. Such an electrophotographic process is called "Carlson process".
The photoconductor employing the organic photoconductive material is advantageous over that employing the inorganic photoconductive material with respect to the degree of freedom in the wave range of the light to be employed, and the film-forming properties, flexibility, transparency, productivity, toxicity, and manufacturing cost of the photoconductor. In light of the above-mentioned advantages, most of the current photoconductors employ the organic photoconductive material.
The photoconductor which is repeatedly operated by the above-mentioned electrophotographic process or the like is required to exhibit excellent electrostatic properties, more specifically, excellent photosensitivity, acceptance potential, retentivity of charge, potential stability, residual potential, and spectral sensitivity.
In recent years, the development of data processing apparatus employing the above-mentioned electrophotographic process is remarkable. In particular, there is a remarkable improvement in the printing quality and the reliability of the digital printer which is capable of recording data by digital recording method, to be more specific, converting the data into digital signals and recording the data using a light. Such a digital recording system is applied not only to the printer, but also to the copying machine. Thus, the digital copying machine is actively developed. It is supposed that the demand for the digital copying machine will further increase in line with the addition of various data processing functions.
The photoconductor designed for the above-mentioned digital recording system is required to have special characteristics which are different from those required for the conventional analogue recording system. For instance, semiconductor laser (LD) or light emitting diode (LED) is widely employed as a light source for the digital recording system because of its compactness, cheapness and high reliability. The wave range of the currently used LD is within the near infrared region, and the wavelength of the currently used LED is 650 nm or more. Therefore, the electrophotographic photoconductors for use with the above-mentioned digital recording system are required to show sufficient sensitivity in the wavelength range from the visible region to the near infrared region.
In light of the above-mentioned sensitivity, a squarylium dye (Japanese Laid-Open Patent Applications 49-105536 and 58-21416), a triphenylamine trisazo pigment (Japanese Laid-Open Patent Application 61-151659), and a phthalocyanine pigment (Japanese Laid-Open Patent Applications 48-34189 and 57-14874) are proposed as the photoconductive materials for use in the digital recording.
In particular, the phthalocyanine pigment, that is, a titanyltetraazaporphyrin compound, can show absorption and photosensitivity in the relatively long wavelength range. In addition, a variety of phthalocyanine pigments can be obtained according to the kind of central metal or the type of crystalline form. Therefore, research and development of this type of phthalocyanine pigment has been actively conducted to obtain the improved photoconductive material for use with the digital recording.
Examples of the conventional phthalocyanine pigments capable of showing good sensitivity include .epsilon.-type copper phthalocyanine, x-type metal-free phthalocyanine, .tau.-type metal-free phthalocyanine, vanadyl phthalocyanine, and titanyl phthalocyanine.
To be more specific, titanylphthalocyanine pigments with high sensitivity are proposed in Japanese Laid-Open Patent Applications 64-17066, 3-128973 and 5-98182. Those titanylphthalocyanine pigments exhibit maximum absorption in the wavelength range of 700 to 860 nm, so that they can show remarkably high sensitivity with respect to the semiconductor laser beam. However, when each of the above-mentioned titanylphthalocyanine pigments is employed in the electrophotographic photoconductor, there still remain a lot of practical problems, for example, decline in charging performance due to fatigue, and temperature- and humidity-dependence of the charging characteristics although the sensitivity is sufficient. This is reported in Y. Fujimaki, Proc. IS&Ts 7th International Congress on Advances in Non-Impact Printing Technologies, 1,269 (1991); K. Diamon et al.; J. Imaging Sci. Technol., 40,249 (1996).
Japanese Patent Nos. 2637487 and 2637485 disclose tetraazaporphyrin pigments having a heterocycle such as pyridine or pyrazine. Further, Japanese Patent Publication No. 3-27111 and Japanese Patent No. 2754739 discloses that a mixture of a phthalocyanine pigment and a pyridinoporphyradine pigment is effective as the photoconductive material.
In addition, a mixture of copper-tetraazaporphyrin compounds obtained by allowing pyridine-3,4-dicarboxylic acid to react with phthalic anhydride is disclosed in Japanese Patent Publication No. 3-27111.
Even though those photoconductive materials are employed in the electrophotographic photoconductor, the above-mentioned requirements for the photoconductor are not satisfied. Namely, the sensitivity in the visible light range and the near infrared range, and the charging characteristics are still unsatisfactory, and in particular, the durability of the photoconductor is insufficient when the photoconductor is subjected to repeated electrophotographic operations.